BIO2146 Animal Anatomy: Invertebrates up to Hemichordates PDF

BIO2146 Animal anatomy: Invertebrates By Concilie Mukamwambali August 2024 Anatomy of invertebrates All animals are heterotrophic multicellular organisms. Animals are divided into two main groups: vertebrates with the backbone, and invertebrates without backbone Invertebr...

BIO2146 Animal anatomy: Invertebrates By Concilie Mukamwambali August 2024 Anatomy of invertebrates All animals are heterotrophic multicellular organisms. Animals are divided into two main groups: vertebrates with the backbone, and invertebrates without backbone Invertebrates are generally soft-bodied animals that lack a rigid internal skeleton for the attachment of muscles They often possess a hard outer skeleton (as in most mollusks, crustaceans, and insects) that serves, as well, for body protection. General anatomy of invertebrtes Epithelium and connective tissue are the two most fundamental forms of animal tissues, and both are found in almost all invertebrates. The extracellular matrix that is secreted by the epithelial cells that make up the epidermis’ outer layer supports the organism. Echinoderms, sponges, and some cephalopods have an endoskeleton developed from the mesoderm. Chitinous exoskeletons in arthropods are derived from the epidermis. Calcium carbonate makes up the shells of brachiopods, polychaeta worms, and some mollusks. While the epidermis of other invertebrates may secrete a variety of surface coatings, such as the pinacoderm of sponges, the gelatinous cuticle of cnidarians, and the collagenous cuticle of annelids, these invertebrates may lack solid structures. Several different cell types, including sensory, glandular, and stinging cells, may be present in the outer epithelial layer.  General anatomy of invertebrates (cont’d) Insects have segmented bodies supported by a hard jointed chitinous exoskeleton. A head, a thorax, and an abdomen are the three separate sections that make up the body segments. Pair of sensory antennae, two compound eyes, one to three simple eyes (ocelli), and three sets of modified appendages that make up the mouthparts are commonly seen on the head. Three pairs of segmented legs one pair for each of the three parts that make up the thorax as well as one or two pairs of wings are attached to the thorax. The digestive, respiratory, excretory, and reproductive systems are all housed within the eleven segments of the abdomen, some of which may be combined. General anatomy of invertebrates (cont’d) The bodily parts of different species vary greatly from one another and have undergone several modifications, particularly the wings, legs, antennae, and mouthparts. Spiders, a group of arachnids, have four pairs of legs and a body made up of an abdomen and a cephalothorax. Spiders don’t have wings or antennas. Since most spiders are venomous, they have mouthpieces called chelicerae that are frequently attached to venom glands. They have a second set of appendages called pedipalps on cephalothorax. These serve as taste and smell organs and are segmented similarly to the legs. Each male pedipalp has a cymbium, which resembles a spoon and serves to support the copulatory organ. I. Anatomy of the sponges Invertebrates include animals like sponges, cnidarians, worms (flat worms, round worms, annelids), mollusks, arthropoda, and echinoderms. Unlike other animals, sponges lack true tissues and organs. Some of them are radially symmetrical, but most are asymmetrical. The sponge is amorphous and often consists of a basket-like structure. Sponges have many small pores at their body surface through which water inters in this animal. They often contains ameba-like feeding cells and spicuoles for support. Flagellated collar cells line the inside of the body wall. II. Anatomy of the cnidarians have two tissue layers (ectoderm and endoderm) The jellylike material between the two layers is called the mesoglea. There are usually cells loosely scattered throughout the mesoglea but not in a defined layer. The basic body plan of all cnidarians consists of the two cell layers enclosing a digestive cavity (gastrovascular cavity) which is the basic internal organ. Possess specialized stinging cells (cnidocytes) borne on the tentacles. Cnidocytes contain fluid-filled capsules (nematocysts) with a harpoonlike coiled thread used for stinging, paralyzing, and capturing prey. Cnidarians have no well-defined separate respiratory, circulatory, or excretory organs. Tentacles surrounding the mouth are used to capture and ingest food. Cnidarians range in size from nearly microscopic to more than 30 m long and more than a ton in weight. Cnidarian body forms A cnidarian may display either the sessile polyp form or the free- swimming medusa form; some pass through both forms during their life cycle. Both possess a hollow cavity with a single opening surrounded by tentacles. The polyp has a basal disc by which it attaches to the substrate; the mouth typically faces away from the substrate. In the medusa (jellyfish) form, the tentacles and mouth face downward.  Cnidarian body forms (cont’d) Some species exists in form of polyp (e.g., coral) or medusa (e.g, jellyfish), other species pass through both forms during their life cycle. For example, an adult and sexual form of hydra is medusa but its juvenile and asexual form is polyp. These forms can also vary depending upon nutritional content of the living environment. Many cnidarian species exhibit this “alternation of generations” meaning that they alternate between polyps and medusae. The cnidarian digestive cavity, called the coelenteron, has a single opening serving as both mouth and anus. This cavity can be either one large chamber, several smaller chambers, or a branched network of canals. The mouth opening is usually surrounded by tentacles. They use cnidocytes on the surface of their tentacles to release nematocysts for attacking and capturing prey. The endodermal lining of the coelenteron, called the gastrodermis, absorbs nutrients derived from the digestive process.  Cnidarian body forms (cont’d) Cnidarians have a decentralized nervous system, muscle tissue, reproductive tissues, and a hydrostatic skeleton. Hydrostatic pressure allows the animals to hold their shape without bones or solid skeleton. The force of muscle contractions against the hydrostatic pressure allows the organism to move. The primitive nervous system of cnidarians consists of a nerve net capable of sensing touch. There is no circulatory or respiratory system within cnidarians. Both circulation and respiration occur by simple diffusion. Reproductive tissues are usually located in the mesoglea of polyps or in the gastric cavity of medusae. Anthozoan anatomy Species within the class Anthozoa include the corals and sea anemones. Most anthozoan species are polyps & do not exhibit alternation of generations. As polyps, the Anthozoa are primarily sessile. Most of anthozoan species live in a colony made up of many small, interconnected anthozoan polyps. These colonies form by asexual reproduction in which the developing bud forms a polyp that remains attached to the parent. Several coral species secrete an exoskeleton. In these species, the ectodermal cells at the base of the polyp secrete the cup- shaped calcous exoskeleton called the calicle or basal plate. As the polyp grows, the calicle size increases and, over time, becomes the major constituent of coral reefs.  Hydrozoan anatomy The class Hydrozoa contains species that include the siphonophores and hydroids. Many Hydrozoa exhibit the alternation of generations between polyp and medusa forms but the polyp form normally dominates in the hydrozoan life cycle. The medusa form is generally small and short-lived. Its primary function is to carry out sexual reproduction and to allow the species to disperse to different locations. Hydrozoa are classified based on the presence of a membrane called the velum that lines the inside edge of the bell in the medusa forms.  Hydrozoan anatomy (cont’d) Hydrozoa of the order Siphonophora can form large & sophisticated colonies of interconnected and interdependent individual polypoids and medusoids called zooids. The term zooid refers to the fact that the individual can usually only function as part of the whole colony: zooids specialized for a particular function usually lose the ability to perform other functions. For example, nectophores are medusoid zooids that function to move the colony through the water. Nectophores however, cannot feed. They are dependent on specialized feeding polyps within the colony to absorb and deliver nutrients. This is a distinguishing feature of the hydrozoans. Hydrozoan anatomy (cont’d) This figure shows the hydrozoan species Portuguese man o’ war. Note the air bladder at the top of the animal and the long feeding tentacles hanging below. The animal floats at the surface of the ocean using that air bladder. Long tentacles, sometimes several meters in length, dangle below the surface where they sting and capture prey.  Scyphozoan and Cubozoan Anatomy The class Scyphozoa (true jellyfish) and Cubozoa (box jellyfish) shows the alternation of generation The difference between these two classes and the Hydrozoa is the predominant form. In most hydrozoan species the polyp form is predominant, with the medusa form being small, short-lived or never detaching from the polyp colony. The opposite is true for the Scyphozoa and Cubozoa. In these classes the medusa stage is generally fairly large, while the polyp stage is small and short-lived. In addition, scyphozoans and cubozoans lack a velum on the inside edge of the bell. Scyphozoan and Cubozoan Anatomy (cont’d) Scyphozoa and Cubozoa are very similar structurally with a few key differences. Box jellies have cube-shaped bodies. There are also differences in the venom present on their nematocysts. One interesting feature of both classes is the presence of small sensory structures called rhopalia. Rhopalia typically have visual structures called ocelli and gravity-sensing structures called statoliths. The ocelli allow both species to sense light. Cubozoa have an even more complex vision system with several sets of eyes that may be capable of seeing blurred images. These eye pairs contain a lens, retina, cornea, and iris. It is not known how clear of an image these eyes are capable of capturing, particularly since the animals lack a centralized nerve system to process the information sensed by the eyes. This feature of cubozoans makes them particularly difficult to observe in their natural habitat because they tend to swim away when they “see” divers approaching.  Anatomy of worms Different and unrelated types of animals that are generally long and soft are called worms. Of these, three common types of worms are: the flatworm (Platyhelminthes) , the roundworm (nemathelminths), and the segmented worm (annelida). Flatworms are soft, unsegmented invertebrates. They do not have specialised respiratory systems. This condition restricts them to this flat shape to allow them to breathe through their skin. Flatworms have only one body cavity through which they eat and excrete waste., but some of them do not show any body cavity. Roundworms (ex. Ascaris) are very smooth and tubular, and have openings on both ends of their bodies (mouth & anus). Segmented worms (ex. Earthworm) have body segments and many have parapodia: leg-like protrusions helping in movement.  III.1. Platyhelminthes anatomy Flatworms have three embryonic tissue layers that give rise to surfaces that cover tissues (from ectoderm), internal tissues (from mesoderm), and line the digestive system (from endoderm). The epidermal tissue is a single layer cells or a layer of fused cells (syncytium) that covers a layer of circular muscle above a layer of longitudinal muscle. The mesodermal tissues include mesenchymal cells that contain collagen and support secretory cells that secrete mucus and other materials at the surface. The flatworms are acoelomates, so their bodies lack body cavity between the body surface and the and the digestive system if any. Some flat worms (the cestodes) do not have a digestive system because they absorbs their food through the skin as the digested food in the intestine of the host flows over and around it.  III.1. Platyhelminthes anatomy (cont’d) Most flatworms, such as the planarian have a gastrovascular cavity rather than a complete digestive system: “mouth” is also used to expel waste materials from the digestive system. The gut may be a simple sac or highly branched.. Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis. Flatworms have an excretory system with a network of tubules throughout the body with openings to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body. The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerves at the anterior end of the worm, where there may also be a concentration of photosensory and chemosensory cells. III.1. Platyhelminthes anatomy (cont’d) There is neither a circulatory nor respiratory system. The gas and nutrient exchange depend on diffusion and cell-cell junctions. This condition limits the thickness of the body  III.1. Platyhelminthes anatomy (cont’d) Tapeworms or cestodes live in the intestinal tract of the primary host and remain fixed using a sucker on the anterior end, or scolex, of the tapeworm body. The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but contain reproductive structures, both male and female. They lack the digestive tract and have no body cavity. Proglottids are produced at the scolex and gradually migrate to the end of the tapeworm; at this point, they are “mature” and all structures except fertilized eggs have degenerated. Most reproduction occurs by cross-fertilization. The proglottid detaches from the body of the worm and is released into the feces of the organism. The eggs are eaten by an intermediate host. The juvenile worm infects the intermediate host and takes up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycle is completed. Cestode (tapeworm) figure (ex. Taenia) III.1. Platyhelminthes anatomy (cont’d) Trematodes are flat and broad resembles the leaf of a tree or flat fish. They vary in size from the species just visible to the naked eye (like Heterophyes) to the large fleshy fluke (like Fasciola hepatica). Flukes are hermaphroditic (monocious) except for Shistosomes in which the sexes are separated. Have muscular cup-shaped suckers (the structure by which the worm attached to the host): the oral sucker surrounding the mouth (the oral or the anterior sucker) at the anterior end & the ventral sucker (or Acetabulum). The body is covered by integument which often bears spines, papillae or tubercles.  III.1. Platyhelminthes anatomy (cont’d) They have no body cavity, circulatory or respiratory organs. The alimentary system consist of mouth surrounded by oral sucker, a muscular pharynx and esophagus which bifurcates anterior to the acetabulum to form two blind caeca (which reunite in some species), the alimentary canal therefore appears like inverted y and the anus is absent. The excretory system consist of flam cells and collecting tubules which lead to median bladder opening posteriorly. There is a rudimentary nervous system consist of two lateral ganglions in the region of pharynx, connecting by dorsal commissures. From each ganglion arise anterior and posterior longitudinal nerve trunks connected by numerous commissures ,(sense organs are almost lacking). The reproductive system is well developed, the hermaphroditic flukes show self fertilization (though in many species, cross fertilization also occur). In the Schistosomes the sexes are separated but male and female live in close apposition (in copula), the female fitting snugly into the folded ventral surface of the male which form the gynaecophoric canal. Trematode figure  III.1. Platyhelminthes anatomy (cont’d) Flatworms of the Monogenea classis are ectoparasitic and attached by special posteriorly positioned attachment organs to their host's skin or gills. Their anterior end contains apical sensory structures, a mouth with or without accessory suckers and special glands or clamps for attachment. All are hermaphrodite.  III.2. Anatomy of round worms The smallest nematodes are microscopic, free-living species can reach as much as 5 cm, and some parasitic species are larger, reaching over 1 m in length. The body is often ornamented with ridges, rings, bristles, or other distinctive structures. Whereas the rest of the body is bilaterally symmetrical, the head is radially symmetrical, with sensory bristles and, in many cases, solid 'head-shields' radiating outwards around the mouth. The mouth has either three or six lips, which often bear a series of teeth on their inner edges. An adhesive 'caudal gland' is often found at the tip of the tail.  III.2. Anatomy of round worms (cont’d) The epidermis is either a syncytium or a single layer of cells, and is covered by a thick collagenous cuticle. The cuticle is often of a complex structure and may have two or three distinct layers. Underneath the epidermis lies a layer of longitudinal muscle cells. The relatively rigid cuticle works with the muscles to create a hydroskeleton, as nematodes lack circumferential muscles. Projections run from the inner surface of muscle cells towards the nerve cords; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibers into the muscles rather than vice versa.  Digestive system of round worms Tubular digestive system, with openings at both ends. Oral cavity is lined with cuticles, which are often strengthened with structures, such as ridges, especially in carnivorous species, which may bear several teeth. The mouth often includes a sharp stylet, which the animal can thrust into its prey. In some species, the stylet is hollow and can be used to suck liquids from plants or animals. The oral cavity opens into a muscular, sucking pharynx, also lined with cuticle. Digestive glands are found in this region of the gut. In stylet-bearing species, these may even be injected into the prey.  Digestive system of round worms No stomach is present, with the pharynx connecting directly to a muscleless intestine that forms the main length of the gut. This produces further enzymes and also absorbs nutrients through its single-cell-thick lining. The last portion of the intestine is lined by a cuticle, forming a rectum, which expels waste through the anus just below and in front of the tip of the tail. The movement of food through the digestive system is the result of the body movements of the worm. The intestine has valves or sphincters at either end to help control food movement through the body.  Excretory system of round worms Nitrogenous waste is excreted in the form of ammonia through the body wall without any any specific organs. However, the structures for excreting salt to maintain osmoregulation are typically more complex. In many marine nematodes, one or two unicellular 'renette glands' excrete salt through a pore on the underside of the animal, close to the pharynx. In most other nematodes, these cells are replaced by an organ consisting of two parallel ducts connected by a single transverse duct. This transverse duct opens into a common canal that runs to the excretory pore  Nervous system (NS) of round worms On anterior end, a dense circular nerve ring serving as the brain surrounds the pharynx. From this ring six labial papillary nerve cords extend anteriorly, while six nerve cords; a large ventral, a smaller dorsal and two pairs of sublateral cords extend posteriorly. Each nerve lies within a cord of connective tissue lying beneath the cuticle and between the muscle cells. The ventral nerve is the largest, and has a double structure forward of the excretory pore. The dorsal nerve is responsible for motor control, while the lateral nerves are sensory, and the ventral combines both functions. The NS is also the only place in the nematode body that contains cilia, which are all nonmotile and with a sensory function. Nematodes are covered by numerous sensory bristles and papillae that together provide a sense of touch. Behind the sensory bristles on the head lie two small pits, or 'amphids'. These are well supplied with nerve cells and are probably chemoreception organs. Some aquatic species possess pigmented eye-spots, but their sensory function is unclear.  Reproductive system of round worms Most nematode species are dioecious, with separate male and female individuals. Reproductive system is tubular. Generally reproduction is sexual. Usually males are smaller than females or hermaphrodites (often much smaller) and have a characteristically bent or fan-shaped tail. During copulation, one or more chitinized spicules move out of the cloaca and are inserted into the genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. Some species, such as Caenorhabditis elegans, are androdioecious, consisting of hermaphrodites and rare males. One or two tubular gonads are present in both sexes. Fig. of male and female nematoda  III. 3. Anatomy of Annelids Triploblastic protostomes with coelom within which the gut and the other organs are suspended. The body cavity is separated into a series of compartments separated by the wall called as septa. In most of the species each compartment is considered as a single segment which encompasses a part of the nervous and circulatory system making them to function independently. Annuli or ringlike structure marks each segment. Each segment has a circular muscle as an outer layer below the thin cuticle and the epidermis layer, and a longitudinal muscle system. Annelids have closed circulatory system, true segmentation, & bilateral symmetry.  III. 3. Anatomy of Annelids (cont’d) Most of annelids carry bristles called setae and parapodia, a pair of appendages. In front of the true segment is the prostomium and peristomium that carry the mouth. At the back (posterior) of animal lies the pygidium, the place where the anus is located. The digestive system is quite uneven but it is specialized. The digestive system is separate from the vascular system and the nervous system. The vascular system contains a dorsal vessel that conveys the blood towards the front part of the worm. It also includes a ventral longitudinal vessel that conveys the blood in the opposite direction. The dorsal vessel and the ventral longitudinal vessel are linked by the vascular sinus and by different kinds of lateral vessels.  III. 3. Anatomy of Annelids (cont’d) The lateral nerves rise in each segment from the solid ventral nerve cord that is located in the nervous system. Every segment in the species unites to perform a single function like locomotion. Growth in many species is done by duplicating some individual segments and in other species the number of segments is fixed early in the development stage itself. Anatomy of Lumbricus terrestris (earthworm) The cuticle of earthworm is a layer of collagen fibers over the epidermis. Earthworms don’t have lungs or other specialized respiratory organs, so they use their epidermis to breathe using diffusion. You’ve probably noticed that worms are made of several rings or segments. The worm’s interior is also divided into segments by septa. Septa keep fluid from passing into other segments; this fluid distribution helps the worm move. Most segments include nephridia, organs which remove waste from the body. Earthworm model showing metanephridia  Anatomy of Lumbricus terrestris (cont’d) Beneath the epidermis and a layer of connective tissue lies the circular muscle layer, which runs in circles around the worm and contracts to make the worm thinner. The longitudinal layer runs from the anterior to the posterior end, and when it contracts, it makes the worm shorter. These muscle tissues help worms move. Located throughout the epidermis are bristles called setae, which allow the worm to gain traction and to move easily in the soil. The worm’s central nervous system is made up of three parts: the brain, the subpharyngeal ganglion, and the ventral nerve cord. The brain is at the top of the digestive tract on the anterior end of the worm. The ventral nerve cord runs down the bottom of the worm’s body and connects to the brain by the subpharyngeal ganglion. Model of the NS of annelids  Anatomy of Lumbricus terrestris (cont’d) Earthworms have five aortic arches that pump blood through the body. They are located around the esophagus on the anterior end of the body, and they connect the ventral and dorsal blood vessels. The dorsal blood vessel pumps blood from the posterior end to the anterior end while the ventral blood vessel pumps blood from the anterior end to the posterior end. As they move through the soil, earthworms take in food with their mouths. After swallowing with their pharynx, the food moves through the esophagus and then is stored in the crop. Then the food moves into the gizzard, where it is ground down more finely by stones then moves into the intestine. In the intestine, food is digested and nutrients absorbed. An earthworm’s intestine is over two-thirds of its body length! After food makes its way through the intestine, waste passes through the anus. Earthworm is hermaphrodite but make cross-fertilasation. The clitellum secretes mucus to form a capsule that protects the worm’s zygotes. Lumbricus terrestris (earthworm): flashcard made with visible biology  IV. Anatomy of mollusks Have a vast range of morphology. The muscular foot is the primary locomotive component. The visceral mass houses the majority of the internal organs. Depending on the type of mollusk being investigated, the foot's structure and function can differ. It serves in movement and anchoring. The leg of molluscs without shells is often the same size as the entrance of the shell. The leg can be extended farther. The foot of members of the class Cephalopoda, which means "head-foot," is funnel-shaped to rapidly discharge water from the mantle cavity, and the front border of the leg has been altered to form a circle of arms and tentacles. Mollusks are the first animals to have evolved organ systems for respiration and circulation. Anatomy of mollusks (cont’d) Mantle and radula are unusual organs found in mollusks, but some mollusks lost them during evolution. circulatory and respiratory systems, digestive system, nervous system, excretory system, and reproductive system are shown on the diagram.  Anatomy of mollusks (cont’d) Mantle and Radula The mantle is a soft tissue layer that lies beneath the shell where it covers the body of the animal. The outer cells of the mantle secrete layers of calcium carbonate that form the shell. A second important role of the mantle is in the formation of a cavity called the mantle cavity. The mantle cavity is formed between the mantle tissue and the body of the animal. This cavity serves as a water pumping station for aquatic mollusks. It contains gills for respiration and exit pores for the digestive, excretory, and reproductive systems. The mantle cavity is also where gametes are released to be dispersed by the out-flowing water.  Anatomy of mollusks (cont’d) Filter feeders, such as bivalves, also use the intake water of the mantle cavity to obtain food. In cephalopods, the mantle cavity has been adapted for use in locomotion. Water is expelled from the cavity through an organ called the siphon with great jet-propulsive force. This force allows some cephalopods to move with rapid speed. The radula is a specialized muscular feeding organ that contains teeth made of a carbohydrate (chitin) substance. It is located in front of the mouth in the head region of all mollusk classes except Bivalvia.  Anatomy of mollusks (cont’d) Respiratory and Circulatory Systems The mollusk circulatory system uses a heart to pump blood through the organism, and the respiratory system of aquatic mollusks centers around their gills. Mollusk gills are ctenidia, and they are made up of a series of thin filaments of tissue that resemble the teeth of a comb. Terrestrial mollusk species have primitive lungs that absorb oxygen directly from the air around them. The gills are located in the water-filled mantle cavity. All mollusks except those in the class Cephalopoda have an open circulatory system. The heart pumps blood through blood vessels that lead from the gills into body cavities called hemocoels. In hemocoels, blood is outside the blood vessels Anatomy of mollusks (cont’d) On schematic representation of an open circulatory system showing blood flow from the gills through blood vessels (in red) and the heart into the hemocoel, black arrows show the flow of blood.  Anatomy of mollusks (cont’d) Excretory System It is made up of tubular organs called nephridia that filter waste from internal body fluids. The nephridia are small tubes that open into the coelom. They have tiny cilia, that surround the tube openings and cause fluid to flow from the coelom into the nephridia tubules. Once the fluid enters the nephridia, non-waste molecules, such as sugars and water, can be reabsorbed into the animal’s body. What remains in the tubes is concentrated waste that is then excreted out of the nephridia exit pores in the mantle cavity.  V. Arthropods anatomy Arthropod animals are grouped into 5 main groups: arachnids, crustaceans, myriapods, hexapods, and trilobites. Arachnids have 8 legs and include spiders and scorpions. Crustaceans are aquatic organisms with 10 legs which include lobsters and crabs. Myriapods are terrestrial organisms with many legs and include millipedes and centipedes. Hexapods are organisms with 6 legs and are common insects like flies, bees, and wasps. Trilobites are extinct marine organisms  V. Arthropods anatomy (Cont’d) Arthropods show a segmented body plan that can typically be divided up into three different sections: head, thorax, and abdomen. The head has many sensory appendages while the thorax can have wings. Some species can have some segments fused together as a cephalothorax. The arthropods are so named because they have jointed legs. An exoskeleton which is a waxy outer covering made of chitin covers the segments. It is molted as the organism grows. V. Arthropods anatomy (Cont’d)  V. Arthropods anatomy (Cont’d) The digestive system of arthropods In general, the Arthropoda digestive system consists of three parts. The foregut is first to bring the food into the body. The midgut is the next section and typically consists of the stomach. The last section is the hindgut consisting of the colon and anus to take waste out of the body. Circulatory system of arthropods Arthropods possess an open circulatory system consisting of a dorsal heart and a system of arteries that may be very limited (as in insects) or extensive (as in crabs). The arteries deliver blood into tissue spaces (hemocoels), from which it eventually drains back to a large pericardial sinus surrounding the heart.  V. Arthropods anatomy (Cont’d) Nervous system of arthropods The central nervous system of arthropods consists in principle of a double chain of ganglia, connected laterally by commissures, and longitudinally by connectives. These latter consist of the axons of neurons whose cell bodies usually lie in the periphery of each ganglion. The double chain of ganglia forms the ventral nerve cord. In living arthropods, three ganglia mark the three-part brain condensed together to form a solid mass in the head of arthropod animal. V. Arthropods anatomy (Cont’d)  V. Arthropods anatomy (Cont’d) Excretory system of arthropods Organs of excretion in arthropods are Malpighian tubules. Malpighian tubules are observed in insects, myriapods, and some arachnids such as spiders and mites. They are slender tubes normally found in the posterior regions of alimentary canal at the junction of the midgut and hindgut. The precise number of Malpighian tubules in an arthropod animal vary from just a few to several hundreds. These excretory structures have a closed end, which lies in the fluid- filled cavity (haemocoel), and an open end, which opens into the gut between the midgut and the rectum. V. Arthropods anatomy (Cont’d): Malpighian tubules  V. Arthropods anatomy (Cont’d) Crustaceans and most of arachnids possess paired excretory organs (maxillary, antennal, or coxal glands) that open at the bases of certain appendages. The green gland consists of a closed ending sac called the end-sac (coelomic sac) connected to an involuted tubule, the labyrinth, leading to the nephridial canal, which terminates in a structure called the bladder. The bladder opens to the external environment via an excretory pore situated at the base of the antenna. The end-sac is surrounded by coelomic fluid which is filtered to produce the initial urine within the gland. Arthropods anatomy (Cont’d): green gland Arthropods anatomy (Cont’d): green gland Respiratory system of arthropods Among the arthropods, -branchiopods, Cirripedia, chironomus larvae, and minute arachnids- respire through the general surface of the skin by diffusion process, but most arthropods have specialized organs for aerial and aquatic respiration. Terrestrial arthropods possess tracheae and book lungs as respiratory organs. Tracheae are a system of tiny tubes that permit passage of gases into the interior of the body. The gills are the respiratory organs of aquatic arthropods. The gills are best developed in crustaceans. In some crustaceans such as crabs and lobsters, the gills are usually derived from the thoracic or occasionally, abdominal appendages and are usually enclosed within the carapace. In other arthropod, special types of gills are encountered. Arthropods anatomy (Cont’d): respiratory argans  Arthropods anatomy (Cont’d) Hormonal system of arthropods Invertebrate endocrine systems use a variety of types of hormones, including steroids, peptides, simple amides, and terpenes. The most well-studied hormone systems are the molting and juvenile hormones in insects, the molting hormones in crustaceans, and several of the neurohormones in mollusks and arthropods. The organizations of arthropod endocrine systems parallel those of the vertebrate endocrine system. That is, neurohormones are produced in the arthropod brain (analogous to the vertebrate hypothalamus) and are stored in a neurohemal organ (like the vertebrate neurohypophysis).  VI. Anatomy of Echinodermata Echinoderms have pentaradial symmetry, a spiny skin, a water vascular system, and a simple nervous system. They lack any kind of central nervous system or brain, but have a nerve ring. Echinoderms also have calcium carbonate endoskeletons, ranging from microscopic spicules in sea cucumbers to visible plates in sea stars and urchins. The spines may be moved by small muscles, but they can also be locked into place for defense. In some species, the spines are surrounded by tiny stalked claws called pedicellaria, which help keep the animal’s surface clean, protect papulae used in respiration, and sometimes aid in food capture. Each arm or section of the animal contains different structures: for eg. digestive glands, gonads, and the tube feet that are unique to the echinoderms. In echinoderms like sea stars, every arm bears 2 rows of tube feet on the oral side, running along an external ambulacral groove. These tube feet assist in locomotion, feeding, and chemical sensations, as well as serve to attach some species to the substratum. VI. Anatomy of Echinodermata (cont’d) This diagram of a sea star shows the pentaradial pattern typical of adult echinoderms, and the water vascular system that is their defining characteristic. Most echinoderms have a complete digestive system and a large coelom. VI. Anatomy of Echinodermata (cont’d) Water Vascular and Hemal Systems Echinoderms have a unique ambulacral (water vascular) system, derived from part of the coelom, or “body cavity.” The water vascular system consists of a central ring canal and radial canals that extend along each arm. Each radial canal is connected to a double row of tube feet, which project through holes in the endoskeleton, and function as tactile and ambulatory structures. These tube feet can extend or retract based on the volume of water present in the system of that arm, allowing the animal to move and also allowing it to capture prey with their sucker-like action. Individual tube feet are controlled by bulblike ampullae. Seawater enters the system through an aboral madreporite (opposite the oral area where the mouth is located) and passes to the ring canal through a short stone canal.  VI. Anatomy of Echinodermata (cont’d) Water circulating facilitates gaseous exchange and provides a hydrostatic source for locomotion and prey manipulation. A hemal system, consisting of oral, gastric, and aboral rings, as well as other vessels running roughly parallel to the water vascular system, circulates nutrients. Transport of nutrients and gases is shared by the water vascular and hemal systems in addition to the visceral body cavity that surrounds the major organs. The NS in these animals is a relatively simple, comprising a circumoral nerve ring at the center and & five radial nerves extending outward along the arms. In addition, several networks of nerves are located in different parts of the body. However, structures analogous to a brain or large ganglia are not present in these animals. Some groups of echinoderms may have well-developed sensory organs for touch, chemo-,photo-, and equilibrio- reception.  VI. Anatomy of Echinodermata (cont’d) A mouth, located on the oral (ventral) side, opens through a short esophagus to a large, baglike stomach. The so-called “cardiac” stomach can be everted through the mouth during feeding (for example, when a starfish everts its stomach into a bivalve prey item to digest the animal—alive—within its own shell!) Podocytes—cells specialized for ultrafiltration of bodily fluids—are present near the center of the echinoderm disc, at the junction of the water vascular and hemal systems. These podocytes are connected by an internal system of canals to the madreporite, where water enters the stone canal. Echinoderms are dioecious, but males and females are similar apart from their gametes. Males and females release their gametes into water at the same time and fertilization is external. The early larval stages of all echinoderms (e.g., the bipinnaria of asteroid echinoderms such as sea stars) have bilateral symmetry. Sea stars, brittle stars, and sea cucumbers may also reproduce asexually by fragmentation, as well as regenerate body parts lost in trauma, even when over 75 percent of their body mass is lost! Sea star anatomy Some people may call them “starfish,” but sea stars are not fish; they are related to sea urchins, sea cucumbers, and sand dollars. Most of them have five arms, though some species can have up to 40. One remarkable thing about sea stars is their ability to regenerate their arms and feet. For larger sea stars, the process of limb regeneration can take up to a year. Some species can regenerate a body from a severed limb. Sea stars have a central disc and arms: the central disc is the circular body from which the arms jut out. Sea stars have an oral surface (the side with the mouth, facing down) and an aboral surface (the side without the mouth, facing up). GIF made with Tours in Visible Biology.  Sea star anatomy (cont’d) Sea stars belong to the phylum Echinodermata, which means “spiny skin” in Greek. Sea stars’ spines protect them from predators. Their skin gills, located at the base of the spines, are evaginations of the skin and are both respiratory and excretory organs. The sea star relies on its endoskeleton for structural support. The endoskeleton is made of calcareous structures called ossicles, connected by soft tissue. Spines project from some ossicles through the epidermis and other ossicles, called plates, provide support. Sea star anatomy (cont’d): Image from Visible Biology.  Sea star anatomy (cont’d) Sea stars do not have blood; they circulate water throughout their body through a network of canals. Respiration occurs through the skin and tube feet. Water enters the body through the madreporite (also known as the sieve plate) on the aboral surface. The madreporite connects to the stone canal, which in turn connects to the ring canal that circles the central disk. The ring canal connects to radial canals that bring water to the arms and branch into lateral canals, which bring water to the tube feet. This system of tubes is mainly for the movement of tube feet but can contribute to respiration. Arrow pointing to the madreporite. Image from Visible Biology. Sea star anatomy (cont’d) The tube feet allow the sea star to move, stay in place, take in oxygen, and release nitrogenous waste. They also allow the sea star to receive sensory information. Each foot is made of an internal ampulla and a podium. The ampulla is the water-filled sac that connects to the lateral canal, and the podium is the muscle that protrudes out of the body.  Sea star anatomy (cont’d) Instead of a brain or a central nervous system, sea stars have a nerve ring that circles the central disk. Radial nerves that run through the arms connect to the nerve ring. At the end of each arm, sea stars have a photosensitive eyespot and their senses of touch and smell come through receptors in the epidermis. The sea star can ingest small particles through its mouth—but that’s not all it can do. Once the tube feet have immobilized or opened the shell of their prey, the sea star can push its cardiac stomach through its mouth into its prey to begin digestion. From the cardiac stomach, food moves into the pyloric stomach inside the body where it is fully digested. Ducts then carry away the waste to the anus, located on the aboral surface. Sea star anatomy (cont’d) Sea star anatomy (cont’d) The pyloric caeca are digestive glands on the aboral side within the arms of the sea star. The pyloric ducts transport the enzymes made in the pyloric caeca into the pyloric stomach. Sea stars reproduce sexually, and a few species can reproduce asexually. Through asexual reproduction, when a sea star has been broken in half, each half can regenerate the missing parts. They can also regenerate from an arm and part of the central disk. Sea stars’ gonads are located beneath the pyloric caeca in each arm. They release either sperm or eggs into the marine environment. Eggs are fertilized externally and hatch into larvae.  VII. Hemichordate anatomy The adult hemichordate worm-like body and body cavities, or coeloms, are divided into three basic parts: the proboscis, collar, and trunk. Stomochord is a hollow protuberance, that arises from the roof of the buccal cavity, called the ‘buccal diverticulum’. It is present in the proboscis. There is no central nervous system: nerve tissue is concentrated in the collar, which is linked with a nervous system in the epidermis, or outer covering. There is an open circulatory system that usually includes a contractile heart-like vesicle, two longitudinal vessels, one dorsal and one ventral, interconnected by lateral vessels and sinuses. Blood is colorless. The pharynx may be perforated by numerous paired gill slits, or they may be absent. Gill slits are dorsal in position.  VII. Hemichordate anatomy (cont’d) The second region of the body, the collar, may bear two or more tentacle-like plumes, which may have a double row of ciliated tentacles well supplied with secretory cells. The network of nerve cells and fibres lying within the epidermis is linked with two main nerve tracts that lie dorsally median (i.e., toward the body midline on the upper side) and ventrally median (on the lower side). The dorsal side of the collar has a neurochord formed by an inpocketing of the epidermis; it may have a central lumen, or cavity, that opens to the exterior anteriorly and posteriorly, or it may have a series of lacunae, or spaces. The neurochord contains large nerve cells, extensions of which reach almost to the tip of the proboscis and into the ventral nerve cord. The general body surface is innervated by a primitive receptor system, which consists of scattered sensory cells. There is no well-defined centre of stimuli and responses. VII. Hemichordate anatomy (cont’d) The digestive tract is complete. The excretory organ is a single glomerulus situated in the proboscis, hence called the proboscis gland. Sexes are separate. Reproduction is mostly sexual. Gonads are one too many pairs. Most of hemichordates develop through a free- swimming tornaria larva.